Sound reproduction devices that operate in accordance with the flexural wave principle are known in the state of the art. Such arrangements are essentially composed of a panel and at least one drive system, where the panel begins to oscillate when sound signals are conducted to the drive system(s). It is characteristic for such sound reproduction devices that a "flexural wave radiation" starts at a critical lower cut-off frequency, where the flexural waves lead to sound being radiated in a frequency-dependent direction along the plane of the respective panel. In other words, a cut through a directivity diagram shows a principal lobe whose direction if frequency-dependent. These relations apply fully to infinitely expanded plates and absorber plates, while the relations for the multiresonance plates treated in this application are clearly more complex because of the strong edge reflexes. This multiresonance plate complexity is due to the fact that the mentioned principal lobe has a number of such principal lobes superimposed on it in a frequency-dependent direction, so that a fan-shaped directivity diagram is created which furthermore is very frequency-dependent. But the multiresonance plates and the absorber plates treated here have in common that the center of their directivity diagram rather points away from the mid-perpendicular. This characteristic allows the room to have a greater effect on the sound wave projection.
The panel is built in accordance with the sandwich principle, where each of two opposite surfaces of a very light core layer are connected to a thin cover layer, for example by means of an adhesive. For the panel to have good sound reproduction properties, the material for the cover layer must have an especially high dilatational wave speed. Suitable cover layer materials are for example thin metal foils or fiber-reinforced plastic foils as well.
The core layer must fulfill special requirements as well. Thus it is necessary for the materials being used to first have a low mass density and low damping. In addition, the core layer materials must have as high a vertical shear modulus as possible with respect to the surfaces that are provided with the cover layers. Finally in the sense of a principal requirement it is necessary for the materials that can be used for the core layers to have a very low modulus of elasticity in the direction in which the subsequently formed core layer has its greatest expansion. These two premises, which at first seem contradictory in reference to the last two requirements, are better fulfilled by a core layer which has a perforated structure with openings of a preferably small cross section extending between the two surfaces to be covered by the cover layers. In addition to the core layers with the perforated structure, hard foams can also be used as the core layer material because these materials have suitable shear and elasticity moduli despite their isotropic properties. In this connection we should not forget to also mention that when hard foams are used as the material for the core layer, the objective of the cover layers is to provide the required anisotropic behavior of the panel.
In order to radiate sound waves by means of a panel as described above, it is necessary to connect the panel to a drive system which produces wave-shaped deformations in the panel that are vertical to the plane of the cover layers. To achieve this the state of the art generally uses magnet systems which are also used for driving conventional loudspeakers. In order for these drive systems to provide the deformation of the panel which is necessary to radiate flexural waves, the drive systems are usually equipped with a corresponding countersupport. This countersupport can for example be formed by a supporting structure which is located away from one of the two cover foils and receives the drive system. Aside from the fact that such a supporting structure not only increases the constructed depth and the weight of such installations, this kind of supporting structure also requires considerable production costs. For that reason the supporting structures which function as a countersupport for the drive systems are now directly connected to the panel. A disadvantage however is that the supporting structures which are connected to the panel impede the generation of flexural waves in the panel and lead to a distorted sound reproduction. This can be attributed to the fact that by comparison with a pure drive system, the attachments required to fasten such supporting structures to the panel stiffen large areas of the panel which extend in the direction of the greatest expansion. It is additionally disadvantageous in such supporting structures that they do not fulfill the properties required by the cover layers and the core layer.
It is therefore the objective of the invention to present a sound reproduction device which eliminates the disadvantages of the state of the art.